"This is a significant milestone in Metrolink's efforts to operate the most efficient and environmentally friendly commuter rail system in the nation," said Board Chair Richard Katz. "It demonstrates our commitment to our neighbors and to doing our part to clean up the air in Southern California."

The first three demonstration locomotives are scheduled to be complete in the fall of 2015, Metrolink said.

In a related move, Metrolink will test alternative fuel sources in future operations. Metrolink will make the first locomotives replaced by the new Tier 4s available to the South Coast Air Quality Management District for the purposes of experimentation with various alternative fuel technologies expected to be commercially available in the next decade.